ABSTRACT Older adults often work harder to understand speech than younger individuals, particularly in background noise. The extra effort that must be devoted to listening may come at the cost of poorer performance on other cognitive and physical tasks and decreased quality of life. Thus, interventions that target listening effort may have widespread public health benefits. Critical to achieving such outcomes is an improved understanding of the mechanisms that contribute to listening effort and thus how it can be comprehensively evaluated and mitigated. The current proposal tests the overarching hypothesis that conflict monitoring and cognitive control neural systems, which support performance in challenging conditions, underlie variation in listening effort with task demands for younger and, to a greater extent, older adults. In two functional neuroimaging studies, participants will be asked to recognize words in background noise while perceptual and response selection difficulty are manipulated (by varying signal-to-noise ratios and multitasking demands, respectively). Subjective report, behavioral performance, and pupillometry measures of listening effort will be concurrently collected. Aim 1 tests the hypothesis that challenging speech recognition engages conflict monitoring and cognitive control neural systems that are sensitive to perceptual and response selection difficulty and differentially indexed by measures of listening effort in younger, normal-hearing adults. Aim 2 tests the hypothesis that increased effort for older, normal-hearing adults are associated with variation in the engagement of these neural systems. The results of this proposal will allow us to both examine which listening conditions are challenging for older adults, and importantly, why they are effortful. This work will thus provide a foundation for future research that aims to develop comprehensive clinical assessments and interventions that target listening effort in older adults with normal to impaired hearing, for whom speech recognition is particularly challenging.